I have been designing an adapter in Eagle and preliminary hand-wired tests prove to work to an extent. It became difficult to test since I only have a working Genesis Model 3 and a Nomad, both of which have the SMS mode disabled. I modded the Nomad to work in SMS mode, but the ROM I tested showed up as a washed out image. Audio worked though. I suspect that I have an address pin stuck somewhere so that the VRAM is receiving garbage data.

The largest supported ROM on a Mega Drive is still 32KBytes, just like the original SMS. This is because the MD contains the same CPU as the SMS. Memory mappers still function as expected, and are required for games larger than 32KBytes.